May 17, 2013 - The use of nonlinear loads such as controlled and uncontrolled rectifiers, cycloconverters, etc., leads to distortion of the shape of the network ...
ISSN 10683712, Russian Electrical Engineering, 2013, Vol. 84, No. 6, pp. 308–313. © Allerton Press, Inc., 2013. Original Russian Text © S.K. Podnebennaya, V.V. Burlaka, S.V. Gulakov, 2013, published in Elektrotekhnika, 2013, No. 6, pp. 15–20.
A Power Parallel Active Filter with Higher Efficiency S. K. Podnebennaya, V. V. Burlaka, and S. V. Gulakov Received May 17, 2013
Abstract—A new approach to expanding the suppression band and reducing the power losses in a parallel active filter by incorporating an additional corrective linear link into its structure, taking into account a delay in the interface filter, and minimizing the switching frequency of inverter power keys is developed. An algo rithm of controlling the corrective link that allows increasing the performance of AF and improving the sup pression of higher harmonics is proposed. Keywords: power parallel active filter, transfer function, interface filter, corrective link, THD DOI: 10.3103/S1068371213060072
The use of nonlinear loads such as controlled and uncontrolled rectifiers, cycloconverters, etc., leads to distortion of the shape of the network voltage curve. This, in turn, leads to a variety of negative conse quences [1], such as increase of losses in electric machines and devices, increase in the intensity of insulation aging, and breach of electromagnetic com patibility. Thus, the task of improving the quality of electricity is important today. One method for improving the quality of electricity in 0.4kV networks is the use of parallel active filters (AFs). Usually, parallel AFs are based on the current inverters (CIs) and voltage inverters (VIs). The classic parallel AF based on the voltage inverter is a VI covered by output current feedback and connected to the network via an interface filter (IF). Control of the power keys of the VI is performed using the pulse width modulation (PWM) [2]. When using parallel AFs based on a VI, the follow ing problems arise: a limited rate of change in output current due to the need to use an IF, and as a result, a limited frequency range of effective compensation of higher harmonics, the presence in the output current of interferences at the switching frequency of the VI that shift the harmonic spectrum toward high frequen cies, which may cause resonance overvoltages in the network [3]. These problems can be solved by increasing the switching frequency of the inverter, consequences of which are a sharp increase in losses in the transistor switches and declining energy efficiency in the AF. To improve the energy performance of the AF, the switch ing frequency of the inverter AF was reduced, thus reducing losses in the keys. At the same time, as the IF, we used an LCLfilter of the third order, the connec tion diagram of which is shown in Fig. 1a, which allows improving the noise cancellation at the switch ing frequency, as evidenced by the increased slope of the response curve of the IF of the third order in the
highfrequency region (Fig. 1b). To calculate the des ignated voltage of the IF, we used spectral methods based on the fast Fourier transformation (FFT), which allowed taking into account the flatness of the IF response curve. Assigned current Iass(jω) that is inserted at the point of the AF connection is calculated under the condi tion that the AF and the load must represent, in rela tion to the network, a symmetric active load with sim ulated resistance R in each phase. This resistance is established from the condition of power balance: 2
2
2
U Arms + U Brms + U Crms R = (2) , P where UArms, UBrms, and UCrms are rootmeansquare phase voltages of the network and P is the sum of active power of the load and power of losses in the AF. The spectrum of current injected by the AF Iass(jω) is calculated as follows: U c ( jω ) I ass ( jω ) = (3) – I L ( jω ), R where Uc(jω) is the voltage range of the network and IL(jω) is the load current range. The components of the AF VI output voltage range are 2
U 1 ( jω ) = U c ( jω ) ( L 1 C 3 ( jω ) + 1 )
(4) 3 + I ass ( jω ) ( L 1 L 2 C 3 ( jω ) + ( L 1 + L 2 )jω ). Equation (4) allows considering the frequency response of the IF in the control system of the inverter. For formation of the voltage inverter signal, a PWM with a triangular carrier is applied. The refer ence voltage for the PWM generator is formed as the inverse Fourier transform of U1(jω). To evaluate the efficiency of the AF control system taking into account the transfer function of the IF, mathematical modeling in a MathCAD medium was
308
A POWER PARALLEL ACTIVE FILTER WITH HIGHER EFFICIENCY (a) Network
L1
ic
iL
309 L2
I1 + I'2
I Load Einv
i2
AF
C3 Ec Ecor
IF L2
L1
U1
C3 VI Fig. 2. Equivalent circuit of power section of AF with con nected CLL.
(b) Conductivity Y, ω dB 50
Response curve of IF of the third order Response curve of IF of the first order
Operating frequency range
–150 1 × 103
1 × 104
1 × 105 1 × 106 Angular frequency ω, rad/s
Fig. 1. Scheme of parallel AF with (a) IF of the third order and (b) response curve of IF of the third order.
carried out; as examples of current receivers, we used the current consumed by a VDG302 welding rectifier loaded on the active resistance, the total harmonic dis tortion (THD) of which equals to 19%. The model parameters of the AF are a carrier fre quency of the PWM of 10 kHz, L1 = 3 mH, L2 = 100 μH, C3 = 47 μF, and a DC link voltage of ±400 V. It is assumed that the rootmeansquare phase voltage of the network is 220 V, there is only the first harmonic in the voltage spectrum, the active filter provides a sin gle power factor, and the network impedance is zero. The simulation results showed that consideration of the transfer function of IF leads to a decrease in the residual THD of the network current by several times. However, changing the parameters of the interface filter and network impedance leads to a reduction in the efficiency of determining the transfer function of the IF of the parallel AF, resulting in insufficient sup pression of higher harmonics of the current and growth of its THD. To improve the efficiency of the AF, we incorpo rated an additional corrective linear link (CLL) into its RUSSIAN ELECTRICAL ENGINEERING
Vol. 84
structure (Ecor, Fig. 2) [4]. The CLL operates in the mode of EMF source and is connected in series with the capacitor of the IF [5]. The main advantage of this inclusion of the CLL is the possibility of reducing the level of its output voltage by an order of magnitude, which allows implementing the power section of the CLL using MOSFET transis tors with better dynamic properties that IGBT ones [4]. Partial current I 2' , which is generated by the EMF of the CLL Ecor and flows in the output circuit of the IF, is set so as to eliminate the regulating error of the main inverter of the AF: I '2 = I ass – I 2 , (5) where I2 is the output current of the AF when operating without CLL and Iass is the current of the task of the AF. When connecting the CLL in series with the capac itor, the transmission coefficient of the IF is defined as follows: L L 2 K ( s ) = I '2 ( s )/E cor ( s ) = L 1 s/ ⎛ L 1 L 2 s + 1 + 2⎞ . (6) ⎝ C 3 C 3⎠ The EDS spectrum of the corrective link E cor ( jω ) = I '2 ( jω )/K ( jω ). (7) The EMF of the corrective link is found as inverse Fourier transform from Ecor(jω). To explore the possibilities of the developed parallel AF, we set up a model simulating its operation (Fig. 3). The model includes the described modifications, in particular the control system (CS) of the main inverter taking into account the transfer function of the IF and adding a CLL operating in the mode of EMF source. The CLL CS has a high sensitivity to changes in the parameters of the IF, resulting in its use being ineffi cient in the case of practical implementation. To solve the problem of high sensitivity of the CLL control system to the parameters of the IF, we devel oped a CLL CS using negative feedback on the output current of the filter. The CLL CS was set up in an ana log form, which increased its reliability and perfor mance due to eliminating the delay introduced by the digital signal processing.
No. 6
2013
310
PODNEBENNAYA et al. Network
iuC(t)
Nonlinear load
iuA(t)
ADC
L2C L2B L2A
+
–
– +
RC
IcorA(jω)
EcorC(jω)
–
IFFT
EcorA
I3C
I3B
I3A
+
I3A(jω) I3B(jω)
FFT
EcorC EcorB
–
I3C(jω)
UC(jω) =
IC(jω) H(jω)
U3A(jω)
U3A
U3B(jω)
U3B
IFFT
U3C(jω)
+ U(jω) A(jω)
(CU)
+
Y(jω) EcorB(jω)
IC(I)
EcorA(jω)
+
Ieid(jω)
Eeid(jω) =
IB(I)
IA(I)
IFFT
–
–
FFT
IC(jω)
IB(jω)
U(jω) I(jω) = R )=
IA(jω)
IcorB
RB
+
IcorC
U2rms P
IcorC(jω)
R=
PC
IcorB(jω)
PB
IcorA
UDC
+ PA
I2C
I2B
I2A
UC(jω)
UB(jω)
ADC UA(jω)
+
+
CC
UC(rms)
UB(rms)
PC
+
RA
CA CB
PB
UA(rms)
PI regulator
+
PA
+
FFT
L1C L1B L1A
i2C(t)
RMS
n
i2B(t)
P = ΣU I
i2A(t)
UA UB UC
PWM
IA IB IC
PDC
i2A(t)
uA(t) uB(t) uC(t)
iuC(t)
i2B(t) i2C(t)
iuB(t) iuB(t)
iuA(t)
U3C
DC VT1
VD3
VD1 VT3
VT5
VT7 VD5
UDC
UDC2
VD7
+ –
+ – –
VT2
VD2
VT4
VD4
VT6
VD6 VT8
VD8
VT9
VT11
VT13
VT10
VT12
VT14
+ –
+ – – DC
Control unit (CU)
PWM U3A
U3B
EcorA
U3C
EcorB
EcorC
Fig. 3. Model of AF CS with corrective link.
The frequency characteristics of the projected CLL CS are shown in Fig. 4, which indicates that they have a “flat” form in the working range.
To minimize CLL power losses, it is powered by an adjustable dc/dc converter with a bipolar output. The CLL power supply is set at that minimally required by
RUSSIAN ELECTRICAL ENGINEERING
Vol. 84
No. 6
2013
A POWER PARALLEL ACTIVE FILTER WITH HIGHER EFFICIENCY Amplitude, dB 30
Phase, deg Response curve
20
200 Phase frequency characteristic 100
10 0
0 Operating range
–10 10
1 × 103 1 × 104
100
1 × 105
–100 ω, rad/s
Fig. 4. Frequency characteristics of CLL control system.
311
need to conserve the linear mode of its output stage operation. This converter also allows symmetrizing the voltages on the dc link capacitors of the main inverter. Experimental verification of the efficiency of the AF with a corrective link was carried out in a labora tory. The AF was made up of three identical phase modules with a common measuring unit and voltage regulation for the capacitors of the dc link imple mented on an ATtiny13 microcontroller. The simpli fied diagram of the AF for one phase is shown in Fig. 5. The circuit comprises a power halfbridge con verter formed by IGBT VT1 and VT2 type Network
Nonlinear load inA(t)
inA(t)
uA(t) To other phases
Voltage divider
ADC
CBS
MK ATmega 168
Voltage divider
+
VT1 Driver TC4420
SPI
MK2 ATtiny 13
ADC
PWM
ADC
PWM
Lowpass filter
CDCP
FGA25N120 L1
L2 M
VT2 C3
Driver HCPL3120 FGA25N120
Voltage divider
DAC MCP4911
+ CDCN
Regulator MCP601
Amplifier TDA7293
Fig. 5. Block diagram of control unit of parallel AF and its power section for one phase. RUSSIAN ELECTRICAL ENGINEERING
Vol. 84
No. 6
2013
L3
312
PODNEBENNAYA et al.
FGA25N120ANTD transistors with a valid current up to 25 A, for control of which TC4420 and HCPL3120 microcircuit drivers are used in a typical circuit activa tion regime. The control signals are generated by an ATMega168 microcontroller (MC). Formation of a PWMsequence with a currentless pause is carried out using a hardware 16bit TIMER1 timer that operates in the Phase Correct PWM mode with a triangular carrier. The analog current signal of the assigned cur rent of the CLL from the controlling MC is formed using a 10bit digital to an MCP4911 analog converter (DAC) connected to the MC via an SPI interface. The current regulator of the corrective link is built on an MCP601 operational amplifier. A TDA7293 amplifier was used as the power section of the CLL. A TZ75L2 current transformer loaded on the negative impedance converter (NIC) was used for measuring the load current [6]. The AF was tested in modes with enabled and dis abled corrective links. Oscillograms of the currents when using the filter are shown in Fig. 6. As a nonlin ear load, we used a sixpulse rectifier with resistive load. The measurements were carried out using a Bry men BM157 current clampon wattmeter and an AKIP4113/1 digital oscilloscope. The experimental results confirmed the correct ness of the theoretical statements described in this work and showed the agreement of the AF operation parameters with predicted ones.
(a) uc(t) iL(t)
(b)
(c)
(d)
CONCLUSIONS (1) A new approach to expansion of the suppres sion band and reduction of power losses in an parallel active filter are developed and scientifically grounded by incorporating an additional corrective link into its structure, taking into account a delay in the interface filter, and minimizing the switching frequency of the inverter power keys. (2) An algorithm for controlling the additional cor rective link that allows increasing the performance of the active filter and improving the suppression of higher harmonics is proposed.
(e)
REFERENCES
Fig. 6. Oscillograms of (a) load current (PWMIL = 29% and network voltage PWMUC = 2.5%); (b) output current of AF without PLL; (c) output current of AF with CLL; (d) network current when operating without CLL, PWMIc = 6.7%; and (e) network current when AF works with CLL PWMIC = 2.9%.
1. Grigor’ev, O., Petukhov, V., Sokolov, V., and Krasilov, I., Higher harmonics in 0.4 kV electric systems, Novosti Elektrotekhn., 2002, no. 6(18). 2. Gaiceanu, M., Active power compensator of the cur rent harmonics based on the instantaneous power the ory, The Annals of “Dunarea de jos” University of Galati: Electrotehrtics, Electronics, Automatic Control, Infor matics. Fascicle III, 2005.
RUSSIAN ELECTRICAL ENGINEERING
Vol. 84
No. 6
2013
A POWER PARALLEL ACTIVE FILTER WITH HIGHER EFFICIENCY 3. Burlaka, V.V., Gulakov, S.V., Bublik, S.K., and D’yachenko, M.D., Parallel active filter with increased suppression coefficient of current higher harmonics, Vinik Priazovs’k. Derzhavn. Tekhn. Univ.: Zb. Nauk pr. Mariupol’, 2009, issue 19. 4. Burlaka, V.V., Gulakov, S.V., Bublik, S.K., and D’yachenko, M.D., Ukraine Patent 93579, 2011. 5. Podnebennaya, S.K., Burlaka, V.V., and Gulakov, S.V., The way to increase efficiency of active parallel power filter by connecting correcting linear link, Avtomatika ta elektrotekhnshka: Matershali Vseukrains’koi nauk ovotekhnichnoi konf. molodikh uchenikh ta studetiv z mizhnarodnoyu uchastyu (Proc. AllUkrainian Sci.
RUSSIAN ELECTRICAL ENGINEERING
Vol. 84
313
Tech. Conf. of Young Scientists and Students with International Participation “Automatics and Electrical Engineering”), Nikolaev: Nats. Univ. Korablestroen., 2012. 6. Burlaka, V.V., Podnebennaya, S.K., and D’yachenko, M.D., The way to expanse dynamical range for measuring current transformers of digital sources for relay protection, Visn. Kremenchug. Derzhavn. Univ. im. Mikhaila Ostrograds’kogo, Kremenchug: Kre menchug Univ., 2010, issue 4 (63), part 3.
No. 6
Translated by K. Lazarev
2013
